A number of technologies have been used for solid state imaging devices, including charged coupled device (CCD) arrays and photoconductor on thin film transistor arrays. More recently, use of metal-oxide-semiconductor (MOS) arrays (particularly complimentary metal-oxide-semiconductor (CMOS) arrays) have been used in conjunction with a photoelectric conversion layer to provide a random-access imaging device having a number of beneficial qualities. Further background information concerning such devices may be found in U.S. Pat. No. 5,528,043, entitled "X-ray Image Sensor", and assigned to the assignee of the present invention, the contents of which are hereby incorporated by reference.
FIG. 1 is a stylized diagram of a prior art MOS imaging array. Such structures are conventionally fabricated on a silicon substrate 10, on which MOS or CMOS circuitry 12 of various types is fabricated using conventional techniques. Distinct pixel circuits 14 each defining a picture element are included as part of the circuitry 12. As is known in the art, a contact of each pixel circuit 14 is in electrical contact with a conductive pixel pad 16. For compatibility with conventional processing techniques, the preferred material for the pixel pad 16 is aluminum or an aluminum alloy. Pixel pads 16 are separated from one another by insulating material 18, which may be, for example, SiO.sub.2 or Si.sub.3 N.sub.4.
In a conventional design, a photoelectric conversion layer 20 is overlaid on top of the pixel pads 16. An electrode layer 22 is then formed on top of the photoelectric conversion layer 20. For most applications, the electrode layer 22 is formed of a thin transparent conductive oxide (TCO) material such as indium tin oxide.
In operation, an electric field is applied between the electrode layer 22 and the pixel pads 16. Photons passing through the electrode layer 22 interact with the photoelectric conversion layer 20 and generate electron-hole pairs. Because of the applied electric field, each electron is drawn towards a nearby pixel pad 16. Differences between the amount of charge induced on the pixel pads 16 create differences in the output of the pixel circuits 14, and can be read out as an image, in known fashion.
A problem with the configuration shown in FIG. 1 is that the materials that can be used for the photoelectric conversion layer 20 are limited to those materials that do not adversely interact with the material of the pixel pads 16. Since the pixel pads 16 are typically made principally of aluminum, the material comprising the photoelectric conversion layer 20 and the process for forming or depositing the photoelectric conversion layer 20 must be compatible with aluminum. In practice, this limitation restricts the ability to use materials for the photoelectric conversion layer 20 that must be formed at temperatures in excess of about 200.degree. C.; otherwise, adverse reactions due to direct chemical contamination or "poisoning" of the aluminum or of the photoelectric conversion layer material may occur.
The inventors have determined that it would be useful in a MOS or CMOS imaging array to be able to use a much wider variety of photoelectric conversion materials having desirable characteristics at different wavelengths of incident light, from infrared to x-ray. Accordingly, the inventors have determined that there is a need to prevent contamination of the photoelectric conversion layer material by migration of aluminum contact material within such an imaging device. The present invention overcomes the problems of the prior art.